Domestic alarm systems have been used to alert for the presence of intruders, smoke, fire, water, etc. However it would be of significant advantage to be able to receive an alert for the presence of the flow of electrical current to an appliance, for example to avoid the possibility of causing a fire by accidentally not shutting off a stove burner or oven. Domestic electrical current is typically carried by wires in a cable. Usually pairs of such wires carry equal current in opposite directions. In the past it was necessary to detect such current by intrusive means, such as by connecting an ammeter or other current detector in series with at least one of the wires, or by isolating one of the wires from the cable and measuring the electromagnetic field surrounding one of the wires by using e.g. a galvanometer or a Hall effect device. It was not possible to measure the electromagnetic field caused by currents carried by both wires without isolating the wires since the currents, traveling in opposite directions establish electromagnetic fields which cancel each other.
Isolating the wires to measure the electromagnetic fields or connecting an ammeter or equivalent circuit to the wires directly can be dangerous to the household do-it-yourselfer, and can be costly if professional service persons are hired to install the detection apparatus.
U.S. Pat. No. 5,250,894 issued Oct. 5, 1994, invented by Ronald P. Bridges et al, describes a system for detecting current carried by plural parallel wires. In this system plural Hall effect devices are disposed at fixed equally spaced positions relative to associated wires. Each Hall effect device detects the electromagnetic field associated with the associated closest wire, as well as the reduced electromagnetic field associated with the more distant two wires. The resulting detected currents associated with the more distant two wires are subtracted from the current output from the detector to result in a net detected current from the closest associated wire. This structure is duplicated for each of the Hall effect devices to provide a combined output signal resulting from all the currents in all of the three spaced wires.